Multiplus 2 intermittent generator rejection

Had a similar headache with my setup on the boat last summer — Multiplus 2 randomly deciding it didn't fancy the generator input and just... rejecting it. No obvious pattern, drove me absolutely mad.

Mine turned out to be the AC input voltage tolerance settings in VictronConnect. The Multiplus can be surprisingly fussy about frequency and voltage staying within tight bounds, and generators — even decent ones with AVR — can wobble a bit under load transients. Worth checking your AC input current limit and the UPS function setting too, as that catches people out more than you'd think.

A few things that helped me narrow it down:

• Checked the VRM portal logs for AC input rejection events — timestamps helped spot a pattern
• Tightened up my generator's idle speed slightly (a small carb adjustment)
• Adjusted the AC Low Disconnect and AC High Disconnect thresholds in the ESS/Charger settings

Also worth asking — what BMS are you running? Some BMS units throw a charge inhibit signal that the Multiplus interprets as a reason to stop, which looks like generator rejection but is actually the battery side being awkward. Had a mate with a Fogstar Drift 48V bank chasing his tail for weeks before realising his BMS comms cable was the culprit.

What generator are you running? EU-spec inverter generators tend to play nicest with the Multiplus in my experience — pure sine, stable frequency. The older AVR-only units can be a lottery.

Anyone else cracked this one? Curious whether it's more common with certain firmware versions too — mine seemed worse after a particular update. 🤔

@JimWilson this one's a classic Multiplus 2 rabbit hole — nine times out of ten on my Vivaro setup it came down to the AC input current limit set too low combined with the generator struggling to hold frequency under load.

Worth checking in VictronConnect:

• AC input current limit — bump it up gradually
• Dynamic current limiter — enable this if your genny has soft frequency droop
• UPS function — toggling this off resolved ghost rejections for me

Also, what's your generator's THD rating? Cheap inverter-gennies with high harmonic distortion will cause the Multiplus to reject seemingly at random. The Victron forums have a brilliant thread on acceptable THD thresholds.

Firmware version matters massively here too — anything below 490 on the Multiplus 2 had known AC input stability bugs.

@VivaroWanderer don't leave us hanging on that one mate 😄

@JimWilson I had almost exactly this on my cabin setup two winters ago — generator feeding a Multiplus 2 through a long cable run, and it would randomly throw a strop. Turned out the AC input voltage waveform from my ageing Honda was drifting just enough under load to trigger the Multiplus's quality threshold.

Dug into VE.Configure and widened the AC input voltage window slightly — also dropped the LOM (Loss of Mains) detection sensitivity, which on a genuine off-grid setup is largely pointless anyway.

Worth checking your generator's output waveform with a decent meter under actual load, not just at idle. A tired carburettor will cause more grief than most people realise. What generator are you running?

@VivaroWanderer yes please do finish that sentence, very inconsiderate cliffhanger there 😄

@JimWilson worth checking a few things that caught me out on my garden office install:

• What's the generator's actual output frequency under load? A cheap inverter-generator drifting outside ±2Hz will trigger rejection
• Have you set the UPS function correctly in VEConfigure? There's an ac input sensitivity setting that's surprisingly aggressive by default
• Is your BMS asserting a charge disconnect at the moment of rejection? Easy to conflate the two faults

Also — what generator are you running? Some of the cheaper Chinese units have dreadful voltage regulation on startup transients. The Multiplus 2 is quite fussy about that initial connection window.

Victron's ESS assistant complicates things further if you're running that. What does VRM/VE.Bus Smart show in the logs at the exact rejection timestamp?

@VivaroWanderer genuinely cannot believe you left it there twice now 😄

Had something similar crop up during commissioning of my shepherd's hut build. Turned out my generator's AC frequency was wandering outside the Multiplus 2's acceptance window under load — you can tighten or loosen those tolerances in VE.Config under Grid settings. Worth downloading VE.Configure and having a look at what your actual frequency and voltage thresholds are set to before assuming it's something more sinister.

What generator are you running @JimWilson? Some of the cheaper inverter-gennies produce surprisingly dirty AC that the Multiplus 2 just refuses outright. Also — is the rejection happening under a specific load condition or seemingly random?